(1) Field of the Invention
The present invention relates to a method for synthesizing diamond at a high temperature under a high pressure, wherein diamond crystals are grown by using diamond particles as seeds.
(2) Description of the Prior Art
When diamond is synthesized according to the hydrostatic pressure method, to obtain crystals having a low inclusion content and a good shape, it is important to grow the crystals at temperatures and pressures very close to the phase equilibrium line. Accordingly, the use of diamond particles as seeds is considered effective for controlling the number of nuclei acting as the centers of growth of the crystals.
In synthesizing diamond according to the hydrostatic pressure method, a temperature difference growth method and a film growth method are adopted as means for growing the crystals. According to the former method, a seed and nondiamond carbon such as graphite are made present and a solvent metal having a temperature gradient is interposed therebetween in such a manner that the seed is positioned on the low temperature side of the solvent metal, where a diamond crystal is grown. According to the latter method, nondiamond carbon is dissolved in a thin film of a solvent metal adhering to the periphery of a diamond seed particle and a diamond crystal is grown by utilizing the solubility difference between the nondiamond carbon and the diamond seed particle. In the film growth method, a nucleus may be generated during the synthesis without making the seed present in advance, but if the seed is used, control of the number of nuclei can be easily accomplished. However, even if the number of nuclei can be controlled, when a diamond crystal is grown in a region where the degree of supersaturation of the nondiamond carbon concentration is high, good quality crystals cannot be obtained and the crystal shape is unsatisfactory. Furthermore, the amounts of included impurities such as the solvent metal, nondiamond carbon, and bubbles are increased. In order to reduce the degree of supersaturation, it is necessary to maintain the temperature and pressure conditions in close proximity to the phase equilibrium line on the side of the stable region of the diamond seed particle. However, in an industrial super-high pressure synthesis apparatus, it is very difficult to regularly measure the temperature and pressure in the reaction zone and to control them to the intended values.
A method is known in which a solvent metal and nondiamond carbon are mixed with diamond seeds, the mixture is molded into columns, and the columns are charged in a high pressure apparatus where diamond crystals are grown. However, since it is difficult to disperse the seeds uniformly in the mixture, deviations are readily caused in the growth of the crystals.
A method is also known in which diamond seeds are incorporated into a solvent metal powder and/or nondiamond carbon powder, both the powders are molded into thin sheets, many solvent metal thin sheets and nondiamond carbon thin sheets are laminated alternately, and crystals are grown under predetermined temperature and pressure conditions. However, deviations are readily caused in the growth of the crystals and it is difficult to obtain diamond crystals having a narrow particle size distribution.
Furthermore, there is known a method in which nondiamond carbon is positioned in contiguous relationship to a catalyst metal (i.e., solvent metal), diamond seeds are placed in holes made in the nondiamond carbon rod or disc, and the combination of the nondiamond carbon, the catalyst metal, and the diamond seeds is subjected to pressure and temperature conditions in the diamond-stable region (U.S. Pat. No. 3,423,177). In this method, the diamond seeds are disposed at a relatively large distance between the adjacent seeds. Therefore, deviations are readily caused in the growth of the crystals, it is difficult to control the particle size of the crystals, and the productivity is low. Furthermore, the growth ratio of the crystal as observed during the crystal growth stage is relatively small and the resulting diamond crystals have poor crushing strength and other mechanical strengths.